Method and device for riser tensioning

ABSTRACT

To maintain a riser under tension, the riser can be connected to a gripper ( 5, 12 ) which is connected to vertical toothed racks ( 4 ) that are in constant engagement with gear wheels ( 22 ) which are driven with constant torque, preferably by hydraulic motors ( 9 ) at constant pressure. An apparatus is also described for putting the riser under tension. The apparatus includes vertical guides ( 7 ) in a derrick, toothed racks ( 4 ) in the vertical guides ( 7 ), a gripper ( 5, 12 ) designed for interaction with a riser connected to the racks ( 4 ), and drive units ( 9 ) arranged in the derrick for drive-actuation of the racks ( 4 ) in the vertical guides ( 7 ).

BACKGROUND OF THE INVENTION

The present invention relates to apparatuses for putting risers undertension, heave-compensating rig floors and for guiding equipment througha moonpool.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained in more detail with reference to theattached figures, wherein:

FIG. 1 is a side view of a derrick; FIG. 2 is a front view of a derrick;FIG. 3 is a side view of an apparatus according to the invention; FIG. 4is a front view of an apparatus according to the invention; FIG. 5 is amore detailed side view of an apparatus according to the invention; FIG.6 is a more detailed front view of an apparatus according to theinvention; FIG. 7 is a top view of an apparatus according to theinvention; FIG. 8 is a top view of a derrick looking towards the rigfloor; FIG. 9 shows a detail of the drive gear of the derrick and theapparatus according to the invention; FIG. 10 is a sectional view of theapparatus according to the invention with drive gear andheave-compensated rig floor; FIG. 11 shows a detail of the drive gear ofthe apparatus according to the invention; FIG. 12 is a sectional sideview of the apparatus according to the invention with drive gear andheave-compensated rig floor; FIG. 13 is a top view of a riser gripperplatform according to the present invention; FIG. 14 is a front view ofthe riser gripper platform; FIG. 15 is a side view of the riser gripperplatform.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a derrick 1 comprising a drive mechanism for a top driverotary machine as described in PCT/NO/98/00/30. This drive mechanismwill not be explained in more detail here; instead reference is made tothe aforementioned document in which there is a comprehensiveexplanation thereof. FIG. 2 is a side view of the derrick 1, and herethe drive mechanism 2 with toothed rack 3 is seen more clearly. FIG. 3is a side view of the apparatus according to the invention, whilst FIG.4 is a front view of the same apparatus. The apparatus comprisespreferably four, but at least two, toothed racks 4, which are arrangedsubstantially parallel to one another. The racks are connected to agripper platform 5, preferably via a relatively flexible articulation 6.

FIGS. 5-7 show the apparatus according to the invention in more detail.The toothed racks are arranged to be passed down into a cavity in a partof the derrick structure. In the illustrated case each rack isaccommodated in a respective hollow leg 7, which constitutes a part ofthe lattice work of the derrick. A drive gear 8, which in theillustrated case comprises eight hydraulic motors 9, arranged in pairson each side of each rack 4, is adapted to move the racks 4 in and outof the legs 7. To obtain synchronisation of the eight hydraulic motors9, these may optionally be connected to each other via shafts 10 and 11.Of course, the synchronisation of the eight motors can also be achievedby other means, e.g., hydraulically or electronically.

The four toothed racks 4 are connected to the gripper platform 5 via anarticulation or flexible connection 6. The articulation or flexibleconnection 6 is capable of neutralising forces that arise due to anyminor imprecision in the parallel movement of the racks 4 in and out ofthe legs 7. The gripper platform 5 comprises two flaps 12, which areadapted to grip below a collar on a non-illustrated riser.

The drive gear 8 is preferably located beneath the rig floor 13. The rigfloor 13 itself may optionally also be physically connected to the racks4, e.g., by means of hydraulic keys 14. In this way, the rig floor 13 isable to follow the movement of the gripper platform 5, and can thus beautomatically heave-compensated. In order to adjust the height of therig floor, the rig floor may be provided with hydraulic cylinders 15,thereby eliminating the need to disconnect the hydraulic locks 14 fromand reconnect them to the racks 4 when making minor height adjustmentsof the rig floor 13.

FIG. 8 is a top view of the rig floor 13, and also shows a sectionthrough the derrick structure 1. This figure also indicates the legs 7,into which the racks 4 are passed. When the racks 4 have been passedinto the legs 7, the racks 4 will not be visible above the rig floor,and nor will they represent any danger to personnel or take up space.FIG. 8 also shows a pipe rack 16 and a pipe handling means 17. On therig floor 13 there are also provided rails 18 for a dolly 19, which isdesigned to carry equipment into position above the moonpool.

FIG. 9 is a sectional view of the drive gear 2 for the rotary machine 20(FIG. 2). This figure also shows a hydraulic motor 9 for actuating therack 4.

FIG. 10 is a more detailed sectional view of the drive gear 8 for theracks 4, and shows two of the machinery rooms 21 in which the hydraulicmotors 9 are arranged. The figure also shows the shaft 10 and the end ofthe shafts 11, which connect the hydraulic motors in each machinery room21 to one another to allow them to be run synchronously. Also shown inthis figure is the rig floor 13, which is equipped with four hydrauliclocks 14 and four hydraulic height-adjusting cylinders 15.

FIG. 11 shows a hydraulic motor 9, which via a gear wheel 22 is intoothed engagement with a toothed portion (not shown) of the rack 4. Ascan be seen, the shafts 10 and 11 are equipped with gear wheels 23 and24 at their ends, which gear wheels are in engagement with toothedportions (not shown) of the racks 4. In the illustrated case the shafts10 and 11 are not in direct operative connection with the hydraulicmotor 9, but are in engagement with the racks 4. However, a directoperative connection between the hydraulic motors 9 is also conceivable.

FIG. 12 is a sectional side view of the apparatus according to thepresent invention, in which the gear wheels 22 on the hydraulic motors 9can be seen, as can a shaft 11 and the gear wheels 23 and 24. The rigfloor 13 is also indicated.

FIG. 13 is a top view of the gripper platform 5, whereas FIG. 14 is afront view of the gripper platform 5, and FIG. 15 a side view of thegripper platform 5. The gripper flaps 12 are equipped with a respectivehydraulic cylinder 25, which is adapted to swing the gripper flaps 12between a horizontal and a vertical position. In the vertical position,the gripper flaps allow the riser and other equipment clear passagebetween them. In the horizontal position, the gripper flaps 12 engagewith a riser 30, which comes to rest in the recesses 26 in the gripperflaps 12 (FIG. 13). The recesses 26 together form a substantiallycircular hole, which has a smaller diameter than a collar on the riser,which collar comes to rest on the top of the gripper flaps. The gripperflaps 12 are also equipped with stops 27, which come to bear against thegripper platform 5, so that load taken up by the cylinders 25 is notparticularly large.

In addition to functioning as a tension device for a riser and as aheave compensator for the rig floor, the apparatus according to theinvention is also capable of functioning as a hoist for equipment thatis to be passed through the moonpool on a drill and production ship. Forthis function, the gripper platform 5 may be equipped with a roller orthe like, which can be brought into contact with the moonpool walls, toprevent any risk of the gripper platform striking against the moonpoolwalls, and causing damage. In this way, e.g., a BOP can be moved safelyand securely through the moonpool, it being secured by the gripperplatform throughout the lowering operation and until it is clear of thebottom of the ship.

The hydraulic motors 9 are of a conventional type and equipped with agear box. By operating these at constant pressure when the riser is tobe held under tension, and connecting the pressure via an accumulator,constant tension in the riser may easily be obtained. Owing to theconstant tension, the racks 4 will always follow the wave lo motion, anda derrick that is connected to the racks will therefore also follow thewave motion. In particular when lowering coiled tubing, this will beadvantageous, as the coil can be placed on the rig floor, and the coiledtubing will not be subjected to large loads whilst being lowered.

The toothed racks are preferably designed to be rigid, but for certainpurposes they may conceivably be constructed so as to be hinged.

Motions have shown that the racks under tension will withstand loads ofas much as 300 tonnes without any difficulty.

What is claimed is:
 1. An apparatus for putting risers under tension, characterized in that it comprises vertical guides (7) in a derrick; toothed racks (4) in the vertical guides (7); a gripper means (5, 12) designed for selectable interaction with a riser, connected (6) to the racks (4), and drive units (9) arranged in the derrick for drive-actuation of the racks (4) in the vertical guides (7), the gripper means maintaining the riser under tension when the gripper means interacts with the riser.
 2. An apparatus according to claim 1, characterised in that the gripper means includes a platform (5) having gripper members (12) for interaction with the riser.
 3. An apparatus for putting risers under tension, characterized in that it comprises vertical guides (7) in a derrick; toothed racks (4) in the vertical guides (7); a gripper means (5, 12) designed for interaction with a riser, connected (6) to the racks (4), and drive units (9) arranged in the derrick for drive-actuation of the racks (4) in the vertical guides (7); wherein the gripper means includes a platform (5) having gripper members (12) for interaction with the riser; the gripper members being in the form of flaps (12) supported in the platform so as to be capable of being swung towards one another, each flap having a recess (26) adapted to the half circumference of the riser.
 4. An apparatus according to claim 1, characterized in that the drive units (9) include hydraulic drive motors.
 5. An apparatus according to claim 2, characterized in that a rig floor (13) is capable of being connected to the racks (4).
 6. An apparatus according to claim 1, characterized in that the rig floor is connected to the toothed racks (4) by means of hydraulically actuated keys (14).
 7. An apparatus according to claim 6, characterised in that the rig floor (13) is connected to the hydraulically actuated keys (14) by means of hydraulic working cylinders (15) so that the drilling floor can be height-adjusted relative to the keys.
 8. An apparatus for putting risers under tension, comprising: vertical guides in a derrick; toothed racks in the vertical guides; a gripper connected to the racks and adapted to selectably interact with a riser, the gripper maintaining the riser under tension when selectably interacting with the riser and for not maintaining the riser under tension when not selectably interacting with the riser; and drive units arranged in the derrick for drive-actuation of the racks in the vertical guides.
 9. The apparatus according to claim 8, wherein the gripper includes a plurality of flaps adapted to swing toward each other.
 10. The apparatus according to claim 9, wherein each of the flaps includes a recess adapted to a half circumference of the riser.
 11. The apparatus according to claim 8, wherein the gripper includes first and second flaps, each of the flaps adapted to move between horizontal and vertical positions, the gripper maintaining the riser under tension when each of the first and second flaps is in the horizontal position. 